Rodent swallowing seconds: light pudding (6)
I believe the answer is:
mousse
'light pudding' is the definition.
(I've seen this in another clue)
'rodent swallowing seconds' is the wordplay.
'rodent' becomes 'mouse' (mouse is a kind of rodent).
'swallowing' means one lot of letters goes inside another.
'seconds' becomes 's'.
'mouse' going around 's' is 'MOUSSE'.
(Other definitions for mousse that I've seen before include "Frothy creamy substance for styling hair" , "Whipped cream dessert" , "Aerosol foam used in hair styling" , "hair product" , "Pope's representative" .)